Resin can be cast around a weight, or weight added later. More shipping cost, might need to be packed more carefully to survive shipping, etc., but I think that's solvable. (If nothing else, make the base hollow and have the purchaser add sand or pennies or other mass as available...)

Each piece weighs several pounds, and holds up gigantic medical texts just fine. If you're still in doubt, we can add weight to them on request. Yes, shipping costs would be greater.~Heather @ Dellamorte & Co. www.dellamorteco.com